In most poker games, the two best possible hands are the royal flush and the straight flush. Players who hold either of these two hands in games such as draw poker or Texas hold 'em are almost assured of having a winning hand.
The straight flush involves having five playable cards all of the same suit and in numerical order. For example, the five through nine of hearts would be a straight flush. A royal flush is a straight flush using the four face cards and a 10 (ace through 10).
If two players each have straight flushes, the "higher" hand (i.e. the one with the highest card in the straight flush) will win the pot. The suit used to make the flush has no bearing on the hand.
The odds of drawing a straight flush from the initial deal in a five-card poker hand are one in 72,193.33. Those odds jump to one in 649,740 for a royal flush.
Poker rooms sometimes offer bonuses for players who hit royal flushes.
A person with a royal flush cannot lose a poker hand -- the worst they can do it "split" a pot with another royal flush.